The ways in which plant growth promoting rhizobacteria facilitate the growth of plants are considered and discussed. Both indirect and direct mechanisms of plant growth promotion are dealt with. Th...

The enhancement of plant growth by free-living bacteria

The worldwide increases in both environmental damage and human population pressure have the unfortunate consequence that global food production may soon become insufficient to feed all of the world's people. It is therefore essential that agricultural productivity be significantly increased within the next few decades. To this end, agricultural practice is moving toward a more sustainable and environmentally friendly approach. This includes both the increasing use of transgenic plants and plant growth-promoting bacteria as a part of mainstream agricultural practice. Here, a number of the mechanisms utilized by plant growth-promoting bacteria are discussed and considered. It is envisioned that in the not too distant future, plant growth-promoting bacteria (PGPB) will begin to replace the use of chemicals in agriculture, horticulture, silviculture, and environmental cleanup strategies. While there may not be one simple strategy that can effectively promote the growth of all plants under all conditions, some of the strategies that are discussed already show great promise.

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Plant Growth-Promoting Bacteria: Mechanisms and Applications

Soil bacteria are very important in biogeochemical cycles and have been used for crop production for decades. Plant–bacterial interactions in the rhizosphere are the determinants of plant health and soil fertility. Free-living soil bacteria beneficial to plant growth, usually referred to as plant growth promoting rhizobacteria (PGPR), are capable of promoting plant growth by colonizing the plant root. PGPR are also termed plant health promoting rhizobacteria (PHPR) or nodule promoting rhizobacteria (NPR). These are associated with the rhizosphere, which is an important soil ecological environment for plant–microbe interactions. Symbiotic nitrogen-fixing bacteria include the cyanobacteria of the genera Rhizobium, Bradyrhizobium, Azorhizobium, Allorhizobium, Sinorhizobium and Mesorhizobium. Free-living nitrogen-fixing bacteria or associative nitrogen fixers, for example bacteria belonging to the species Azospirillum, Enterobacter, Klebsiella and Pseudomonas, have been shown to attach to the root and efficiently colonize root surfaces. PGPR have the potential to contribute to sustainable plant growth promotion. Generally, PGPR function in three different ways: synthesizing particular compounds for the plants, facilitating the uptake of certain nutrients from the soil, and lessening or preventing the plants from diseases. Plant growth promotion and development can be facilitated both directly and indirectly. Indirect plant growth promotion includes the prevention of the deleterious effects of phytopathogenic organisms. This can be achieved by the production of siderophores, i.e. small metal-binding molecules. Biological control of soil-borne plant pathogens and the synthesis of antibiotics have also been reported in several bacterial species. Another mechanism by which PGPR can inhibit phytopathogens is the production of hydrogen cyanide (HCN) and/or fungal cell wall degrading enzymes, e.g., chitinase and s-1,3-glucanase. Direct plant growth promotion includes symbiotic and non-symbiotic PGPR which function through production of plant hormones such as auxins, cytokinins, gibberellins, ethylene and abscisic acid. Production of indole-3-ethanol or indole-3-acetic acid (IAA), the compounds belonging to auxins, have been reported for several bacterial genera. Some PGPR function as a sink for 1-aminocyclopropane-1-carboxylate (ACC), the immediate precursor of ethylene in higher plants, by hydrolyzing it into α-ketobutyrate and ammonia, and in this way promote root growth by lowering indigenous ethylene levels in the micro-rhizo environment. PGPR also help in solubilization of mineral phosphates and other nutrients, enhance resistance to stress, stabilize soil aggregates, and improve soil structure and organic matter content. PGPR retain more soil organic N, and other nutrients in the plant–soil system, thus reducing the need for fertilizer N and P and enhancing release of the nutrients.

Soil beneficial bacteria and their role in plant growth promotion: a review

Production of the phytohormone indole-3-acetic acid (IAA) is widespread among bacteria that inhabit the rhizosphere of plants. Several different IAA biosynthesis pathways are used by these bacteria, with a single bacterial strain sometimes containing more than one pathway. The level of expression of IAA depends on the biosynthesis pathway; the location of the genes involved, either on chromosomal or plasmid DNA, and their regulatory sequences; and the presence of enzymes that can convert active, free IAA into an inactive, conjugated form. The role of bacterial IAA in the stimulation of plant growth and phytopathogenesis is considered.

Bacterial biosynthesis of indole-3-acetic acid

1 Although its importance for plant mineral nutrition and nutrient cycling has long been recognized, the soil community has rarely been integrated into dynamical frameworks of plant populations, in spite of abundant evidence for its involvement. The concept of feedback may provide theoretical and experimental tools for investigating the importance of the soil community in the population ecology and evolution of plants. 2 A mathematical model demonstrates the potential for two divergent dynamics, with positive feedback leading to the loss of diversity at a local scale and negative feedback leading to its maintenance. A linear contrast of the growth of plants in association with their own soil communities compared to the growth of plants in association with each others' soil communities can be used to differentiate between these possibilities in empirical studies. 3 Spatially explicit computer simulations demonstrate that the dynamics of a spatially structured community, as the soil community is likely to be, can differ from those predicted for a well-mixed population. Specifically, diversity can be maintained between locally homogeneous patches when positive feedback and dispersal occur at local scales. 4 Using a simple experimental protocol, we have found substantial negative feedback on plant growth through the soil community, suggesting that it may be involved in the maintenance of plant species diversity. 5 We discuss the importance of the soil community in other areas of plant ecology and evolution, including the suggestion that interactions with the soil community may be involved in the maintenance of sexual or asexual reproductive systems.

Incorporating the soil community into plant population dynamics: the utility of the feedback approach.

Azospirillum brasilense, a nitrogen-fixing bacterium found in the rhizosphere of various grass species, was investigated to establish the effect on plant growth of growth substances produced by the bacteria. Thin-layer chromatography, high-pressure liquid chromatography, and bioassay were used to separate and identify plant growth substances produced by the bacteria in liquid culture. Indole acetic acid and indole lactic acid were produced by A. brasilense from tryptophan. Indole acetic acid production increased with increasing tryptophan concentration from 1 to 100 μg/ml. Indole acetic acid concentration also increased with the age of the culture until bacteria reached the stationary phase. Shaking favored the production of indole acetic acid, especially in a medium containing nitrogen. A small but biologically significant amount of gibberellin was detected in the culture medium. Also at least three cytokinin-like substances, equivalent to about 0.001 μg of kinetin per ml, were present. The morphology of pearl millet roots changed when plants in solution culture were inoculated. The number of lateral roots was increased, and all lateral roots were densely covered with root hairs. Experiments with pure plant hormones showed that gibberellin causes increased production of lateral roots. Cytokinin stimulated root hair formation, but reduced lateral root production and elongation of the main root. Combinations of indole acetic acid, gibberellin, and kinetin produced changes in root morphology of pearl millet similar to those produced by inoculation with A. brasilense. Images

Plant Growth Substances Produced by Azospirillum brasilense and Their Effect on the Growth of Pearl Millet (Pennisetum americanum L.)

Polygalacturonic acid transeliminase (PATE) was produced by all of six Azospirillum strains studied. Characteristics were similar to those of PATE from other bacteria: activity was maximal at pH 8.0 and was stimulated by CaCl2. Polygalacturonic acid was used more readily than pectin as a substrate. Polygalacturonic acid in the medium stimulated PATE production by several but not all strains. In all cases some of the PATE produced in cultures remained bound to cell walls. In one strain, most remained cell wall bound. When nitrogen was supplied as amino acids rather than ammonium salts, the ratio of free to bound enzyme was increased. The strains studied varied considerably to response to nutrient amendments and in maximum PATE activity.

T. M. Tien

Papers

Plant Growth Substances Produced by Azospirillum brasilense and Their Effect on the Growth of Pearl Millet (Pennisetum americanum L.)

POlygalacturonic acid transeliminase production by Azospirillum species.